#4.14 dict的迭代
>>> d={'a':1,'b':2,'c':3}
>>> for key in d:         # 用for循环语句迭代字典中的键
...  print(key)
...
a
b
c

#4.15 同时引用两个变量
>>> for x,y in [(1,1),(2,4),(3,9)]:
...  print(x,y)
...
1 1
2 4
3 9

#4.16 三个变量的迭代
>>> for x,y,z in [(1,2,3),(4,5,6),(7,8,9)]: 
...  print(x,y,z)
...
1 2 3
4 5 6
7 8 9